The present invention relates, in general, to a device for detecting and counting broken filaments in a strand made up of a large number of fiber glass filaments, and, more particularly, to a system for counting such filaments over a long period of time or over a long length of the strand to obtain accurate and statistically significant measurements of the number of broken filaments for use in providing an indication of strand quality and for use in controlling the manufacturing process.
Fiber glass strands typically are formed by drawing a large number of individual filaments from apertures formed in a fiber glass bushing, coating the filaments with a suitable binder, and gathering these filaments into strands which are collected on one or more collets to produce forming packages. The process is carefully monitored to maintain filament diameter and integrity during the high speed drawing process and numerous monitoring systems are known for responding to the breakage of filaments to shut down the process. The strands so formed and collected may be used for many purposes; for example, strands may be drawn from the forming packages and twisted together to form a yarn for use in weaving textiles. The twisted strands are rewound from the forming packages onto bobbins which then supply the yarn to weaving looms or the like. Some of this yarn is rewound onto warp beams for use in the production of fabrics, the yarn on such beams then being used as the warp threads in the fabric being woven. Such yarns must be wound carefully and at precisely controlled tensions on the warp beam in order to ensure a high-quality fabric.
In the processing of the filaments into a strand or processing strands into yarn, the twisting and winding operations produce numerous broken filaments at their surfaces. These broken filaments tend to extend out of the strand or yarn at substantially right angles from the axis of the strand, and not only can adversely affect the quality of the fabric woven therefrom, but can affect the operation of a loom using such yarn, as well. For example, such broken filaments appearing in a cloth used in the production of printed circuit boards can produce small irregularities in the circuit board which can result in short circuits on the printed circuit itself.
Broken filaments can also produce problems in nonfabric applications of fiber glass strand. For example, such strands are often used in the manufacture of insect screens, where the strands are coated with a resin, and the coated strand is passed through an orifice to remove excess resin to limit the diameter of the coated product. Broken filaments on such strands can accumulate in the orifices and eventually block them, thereby degrading the quality of the screening.
The breakage of filaments in a strand may be the result of the twisting, rewinding, and other mechanical handling of the strand, and thus the quantity of broken filaments can provide a guide as to whether the handling equipment is operating properly. More importantly, however, the breakage of filaments provides an indication of the quality of the fiber manufacturing process, and, accordingly, the amount of breakage that occurs can be used in the control of the various parameters of a fiber-making process. There are about 30 to 40 variables in this process, including the temperature of the bushings at the orifices, the temperature of the glass, the materials in the melt, and the like. Variations in these parameters can cause very subtle changes in the filaments which can show up as a change in the amount of breakage that is occurring in the strands.
Thus, it is desirable to get an accurate measure of filament breakage in strands, in order to monitor both the manufacturing process and the mechanical handling of the glass so as to enable both the manufacturer of the filaments and the manufacturer of the products made therefrom to provide quality assurances to their respective customers.
The breakage of filaments in a strand has been found to be of an extremely random nature, however, with the number of filament breaks per unit length of strand varying widely not only on a single bobbin, but also between a number of bobbins drawn from the same forming package, or between packages drawn from the same fiber glass melt. The random nature of this breakage makes it very difficult to know with any confidence whether a particular measurement, taken from a relatively short strand length, is anywhere close to the average amount of breakage for the strand on a bobbin, for example, for it is very difficult to even determine what that average amount might be. Accordingly, it has not been possible in the past to determine from measurements of filament breakage whether a change in the manufacturing process or in the handling of the fiber has had any significant effect on breakage, or whether a given measurement is simply within the normal variation to be expected with random distribution. It has been possible to obtain a value for filament breakages over selected lengths of strands with existing measuring devices, but such measurements have been of little value since they were extremely slow, and, therefore, provided statistically insignificant readings which could not realistically be compared to a significant average value, since the latter value was not available. Therefore, although it was known that filament breakage was a problem, and although various devices have been provided in the past to measure the quality of fiber glass strands and yarns, the prior art has not provided a device or system for providing statistically accurate mean or average values of filament breakage, which would permit accurate measurements of this aspect of the quality of the fiber glass strands being provided to a customer.
U.S. Pat. Nos. 3,729,635 and 4,184,769 are examples of prior art devices and systems for detecting defects in yarn through the use of optical sensors. In both patents, the yarn is passed through a sensor, with the output of the sensor varying in accordance with the thickness of the yarn. In accordance with U.S. Pat. No. 3,729,635, if more than a predetermined number of variations, or faults, occurs within a unit time, the winder, which may be a warp beam, stops to allow visual inspection of the yarn. In a similar manner, the device of U.S. Pat. No. 4,184,769 generates a defect signal upon detection of a predetermined number of faults. Devices of this type provide continuous measurements of the variation of thickness of a yarn or strand, and, in order to minimize errors, the light transmitters for such devices must be driven by carefully regulated power supplies, with expensive beam splitters to provide feedback control being utilized in some such units. Furthermore, expensive optics and complex circuitry are required to obtain the degree of accuracy required to insure that the analog output signals are proportional to the thickness of the yarn, and that the system will respond even to very slowly changing conditions which produce essentially a DC output. Such measuring devices, besides being expensive, typically are quite slow, being capable of measuring only about 80 meters of strand per minute. Because of this slowness, it is usual to take samples only at selected points within a bobbin as it is being unwound, with measurement typically being made at three or four points within the bobbin. Because of the random nature of filament breakage, such measurements do not provide an accurate picture of the quality of the strand, but, rather, produce results which are not much improved over a simple visual inspection of the outside layer of a bobbin.